The ever-increasing demand for broadband communication systems has led to optical-transmission systems based on optical waveguides such as fiber optics and optical-processing elements. Generally, in high-performance communication systems, photons continue to supplant electrons as messengers. In defense-related systems, one can encounter threats over a broad spectrum of radio frequencies. The systems need to cover the spectrum with sufficient selectivity to separate simultaneously received signals that are closely spaced in frequency. These requirements can be met through frequency channelization.
Optical wavelengths (or frequencies) can be separated (demultiplexed) or recombined (multiplexed) through physical phenomena known as refraction and diffraction. Many multiplexers (mux) and de-multiplexers (demux) in current use, such as arrayed waveguide gratings (AWGs) mux/demux occupy a large footprint because a large number of distinct waveguide delay-lines (phasar arms) need to be integrated, so that an optical phased array can be formed on a chip. Specifically, the array of phasar delay-lines plays an analogous role as a diffraction grating (e.g., in a spectrometer) in setting the conditions of constructive interference for each incident wavelength. After passage through the phasar arms, the incident wavelengths can be demultiplexed because optical interference causes their phase fronts to propagate in different directions. The wavelength resolution (δλ) of an AWG mux or demux is inversely proportional to the number of phasar arms in the optical phased array. Hence, high channelization resolution is achieved by integrating a large number of phasar arms on the AWG chip.
Other de-multiplexers include traditional dispersive devices, such as diffraction gratings and prisms. While simpler in fabrication and less expensive than AWGs, these devices typically have lower resolution and have a form factor that is too large for insertion in fiber optic links. Because of their operational dependence on free-space optics, these devices are also very sensitive to temperature and generally to vibration/shock, making them unsuitable for insertion or deployment in a fiber optic link.
In conventional frequency channelizers, optical spectrum analyzers (e.g., AWG demux) of the finite impulse response (FIR) design use an array of waveguides (phasar arms) to generate differential phase-shifts that enable different optical frequencies to be focused to distinct output ports lying on an output arc of a planar waveguide coupler, i.e., a slab waveguide coupler.
For example, the passband width and channel spacing of a conventional silica AWG de-multiplexer are typically 0.3 nm (37.5 GHz) and 0.4 nm (50 GHz), respectively. These known AWGs use an array of distinct phasar arms to generate the differential phase-shifts that resolve an incident spectrum. Such a design leads to relatively large chip sizes, such as 30 cm2 or more for 64 channels.
In view of the above-described shortcomings, new methods and design methodologies are needed to provide effective RF-photonic frequency channelizers with reduced chip sizes, such as (for example) 10 cm2 or less. Typically, the fluctuations of optical phase (such as those due to temperature variations) increase with the footprint of the device; therefore, smaller devices would be beneficial for improved stability as well as for practical reasons.
Improved frequency channelizers should also have high resolution, such as the capability to channelize an input optical signal into channels having a passband width of less than 100 MHz, such as 50 MHz or less, and preferably 30 MHz or less.